1. Field of the Invention
This invention generally relates to polymer thick film conductive compositions. In particular, the invention is directed to such compositions, which are suitable for making position sensing elements.
2. Description of the Related Art
Electrically conductive polymer thick film compositions have numerous applications. Polymer thick film (PTF) conductive compositions are screenable pastes, which are used to form conductive elements in electronic applications. Such compositions contain conductive filler material dispersed in polymeric resins, which remain an integral part of the final composition after processing.
Electrically conductive compositions are used as conductive elements in variable resistors, potentiometers, and position sensor applications.
A Position sensor includes one or more voltage indicating position sensing variable resistor elements. These resistor elements are also prepared from polymer thick film materials. The resistive element is in most cases printed over a conductive element which acts as the collector element. In position sensing applications, a metallic wiper slides over the resistive element. The wiper can slide back and forth for several million cycles over the collector and resistive elements during the lifetime of the electronic component.
For accurate position sensing, the wiper should give continuous electrical output throughout the life of the sensor. The durability of these position-sensing elements depends on the mechanical properties of both the resistor and the conductive film. The polymer thick films tend to wearout after several million cycles of sliding with a metallic contactor over the elements at extreme temperature conditions typically seen in an environment such as an automotive engine compartment. Polymer resistive and conductive compositions having excellent mechanical properties are required for performance and signal output in these applications.
In addition to good mechanical properties, these materials should also have good thermal properties. Polymer thick films show a decrease in storage modulus as temperature is increased. A sharp decrease in mechanical properties is observed near the glass transition temperature. In addition to loss in modulus, these materials also tend to show an increase in coefficient of thermal expansion, which increases significantly above the glass transition temperature. A position sensor is exposed to high temperatures in under the hood applications. At these temperatures elements show a high rate of wear due to a decrease in modulus properties. In addition to the surrounding temperature, a still higher temperature is observed at the interface between the metallic wiper and the element surface due to frictional heating. In some cases these temperatures can approach the glass transition temperature (tg) of the material and can cause loss of the mechanical properties, which adversely affect the signal output. Polymer thick film materials prepared from polymers with higher glass transition temperature would be expected to perform better for these applications.
Another important property desired of these materials is a strong adhesion to the substrate as well as to the resistive materials. A loss in adhesion can cause accelerated wear or chipping of the conductive film. In automotive applications, lubricants used in other components may come into contact with the sensor and can diffuse into the interface between the substrate and conductive film. This diffusion of the lubricant fluids can lead to a loss in adhesion of the conductive film to the substrate. A strongly bonded conductive material to the substrate can prevent this diffusion of the lubricant into the interface. For similar reasons as described above, conductive materials should have a strong interfacial bond to the resistive elements.
Substrate materials used in position sensor applications vary from polyimide, phenolic, FRP, ceramic, etc. In order to increase adhesion of the conductive materials to the substrates, some sensor manufacturers plasma treat the substrate surface to create an active surface to bond with the conductive elements. The plasma treatment is an expensive process step and an avoidance of this process step can lead to significant cost savings. Functional groups, which can create strong adhesive bonds with substrates even without a plasma treatment, are preferred for cost saving and other performance requirements.
Flexible position sensing elements such as polymer thick films on polyimide substrates undergo numerous back bending, forward bending, creasing, twisting, and other mechanically harsh process steps. A conductive material of brittle nature can fail during these operations. The cracks as a result of deformation cause a severe decrease in conductivity and other electrical and mechanical properties. A conductive element prepared from a flexible polymer is preferred for these applications.
A smooth surface of the conductive element is desired for improved electrical properties. The position sensing elements are expected to show low linearity deviation before and during the lifetime of these components. A smooth conductive surface contributes to low microgradient. Another requirement for position sensors is low linearity deviation. A highly conductive element would give low linearity deviation
Higher molecular weight of the polymers and low average particle size of conductive particles can contribute to desired rheological properties which results in low surface roughness. Lubricants are generally applied over the resistor and collector elements and tribological properties of the lubricants are often determined by the surface roughness of the resistor and collector elements. A smooth collector surface is desired.
A good processing flexibility is desired for application of a conductive composition onto a variety of substrate materials. A low curing temperature is required for phenolic and epoxy reinforced FRP materials, where as ceramic and kapton substrates can be cured at higher temperatures. It is desirable to have a conductive composition that can be cured at a wide range of temperatures. A short cure time is desirable due to both substrate limitations and processing costs.
Another desirable property for a conductive composition is a long shelf life. A change in viscosity during storage can affect the processability and result in poor printing qualities. This can also lead to position sensing elements with widely varying performance.
A current unmet need exists for a conductive composition that can meet the above mentioned necessary attributes.